System and method for position calculation of a mobile device

ABSTRACT

A system and method for determining a location of a device. Plural signals from a first plurality of satellites may be received a the device where an estimated location of the device is determined as a function of frequency information from the signals. A location of the device may then be determined as a function of the estimated location and/or as a function of phase information from the signals. A second plurality of satellites may also be determined as a function of any one of the determined locations. In such embodiments, assistance data may be transmitted to the device that includes information from the second plurality of satellites. Another location of the device may then be determined from this information.

RELATED APPLICATIONS

The instant application claims the priority benefit of U.S. ProvisionalApplication No. 61/012,319, filed Dec. 7, 2007, the entirety of which isincorporated herein by reference. The instant application is related toU.S. Application Ser. No. 12/099,694, filed Apr. 8, 2008 and U.S.Application Ser. No. 12/050,794, filed Mar. 18, 2008, the entirety ofeach incorporated herein by reference.

BACKGROUND

Radio communication systems generally provide two-way voice and datacommunication between remote locations. Examples of such systems arecellular and personal communication system (“PCS”) radio systems,trunked radio systems, dispatch radio networks, and global mobilepersonal communication systems (“GMPCS”) such as satellite-basedsystems. Communication in these systems is conducted according to apre-defined standard. Mobile devices or stations, also known ashandsets, portables or radiotelephones, conform to the system standardto communicate with one or more fixed base stations. It is important todetermine the location of such a device capable of radio communicationespecially in an emergency situation. In addition, in 2001 the UnitedStates Federal Communications Commission (“FCC”) required that cellularhandsets must be geographically locatable. This capability is desirablefor emergency systems such as Enhanced 911 (“E-911”). The FCC requiresstringent accuracy and availability performance objectives and demandsthat cellular handsets be locatable within 100 meters 67% of the timefor network based solutions and within 50 meters 67% of the time forhandset based solutions.

Current generations of radio communication generally possess limitedmobile device location determination capability. In one technique, theposition of the mobile device is determined by monitoring mobile devicetransmissions at several base stations. From time of arrival orcomparable measurements, the mobile device's position may be calculated.However, the precision of this technique may be limited and, at times,may be insufficient to meet FCC requirements. In another technique, amobile device may be equipped with a receiver suitable for use with aGlobal Navigation Satellite System (“GNSS”) such as the GlobalPositioning System (“GPS”). GPS is a radio positioning system providingsubscribers with highly accurate position, velocity, and time (“PVT”)information.

FIG. 1 is a schematic representation of a constellation 100 of GPSsatellites 101. With reference to FIG. 1, GPS may include aconstellation of GPS satellites 101 in non-geosynchronous orbits aroundthe earth. The GPS satellites 101 travel in six orbital planes 102 withfour of the GPS satellites 101 in each plane. Of course, a multitude ofon-orbit spare satellites may also exist. Each orbital plane has aninclination of 55 degrees relative to the equator. In addition, eachorbital plane has an altitude of approximately 20,200 km (10,900 miles).The time required to travel the entire orbit is just under 12 hours.Thus, at any given location on the surface of the earth with clear viewof the sky, at least five GPS satellites are generally visible at anygiven time.

With GPS, signals from the satellites arrive at a GPS receiver and areutilized to determine the position of the receiver. GPS positiondetermination is made based on the time of arrival (“TOA”) of varioussatellite signals. Each of the orbiting GPS satellites 101 broadcastsspread spectrum microwave signals encoded with satellite ephemerisinformation and other information that allows a position to becalculated by the receiver. Presently, two types of GPS measurementscorresponding to each correlator channel with a locked GPS satellitesignal are available for GPS receivers. The two carrier signals, L1 andL2, possess frequencies of 1.5754 GHz and 1.2276 GHz, or wavelengths of0.1903 m and 0.2442 m, respectively. The L1 frequency carries thenavigation data as well as the standard positioning code, while the L2frequency carries the P code and is used for precision positioning codefor military applications. The signals are modulated using bi-phaseshift keying techniques. The signals are broadcast at precisely knowntimes and at precisely known intervals and each signal is encoded withits precise transmission time.

GPS receivers measure and analyze signals from the satellites, andestimate the corresponding coordinates of the receiver position, as wellas the instantaneous receiver clock bias. GPS receivers may also measurethe velocity of the receiver. The quality of these estimates dependsupon the number and the geometry of satellites in view, measurementerror and residual biases. Residual biases generally include satelliteephemeris bias, satellite and receiver clock errors, and ionospheric andtropospheric delays. If receiver clocks were perfectly synchronized withthe satellite clocks, only three range measurements would be needed toallow a user to compute a three-dimensional position. This process isknown as multilateration. However, given the engineering difficultiesand the expense of providing a receiver clock whose time is exactlysynchronized, conventional systems generally account for the amount bywhich the receiver clock time differs from the satellite clock time whencomputing a receiver's position. This clock bias is determined bycomputing a measurement from a fourth satellite using a processor in thereceiver that correlates the ranges measured from each satellite. Thisprocess requires four or more satellites from which four or moremeasurements can be obtained to estimate four unknowns x, y, z, b. Theunknowns are latitude, longitude, altitude and receiver clock offset.The amount b, by which the processor has added or subtracted time, isthe instantaneous bias between the receiver clock and the satelliteclock. It is possible to calculate a location with only three satelliteswhen additional information is available. For example, if the altitudeof the handset or mobile device is well known, then an arbitrarysatellite measurement may be included that is centered at the center ofthe earth and possesses a range defined as the distance from the centerof the earth to the known altitude of the handset or mobile device. Thealtitude of the handset may be known from another sensor or frominformation from the cell location in the case where the handset is in acellular network.

Traditionally, satellite coordinates and velocities have been computedinside the GPS receiver. The receiver obtains satellite ephemeris andclock correction data by demodulating the satellite broadcast messagestream. The satellite transmission contains more than 400 bits of datatransmitted at 50 bits per second. The constants contained in theephemeris data coincide with Kepler orbit constants requiring manymathematical operations to turn the data into position and velocity datafor each satellite. In one implementation, this conversion requires 90multiplies, 58 adds and 21 transcendental function cells (sin, cos, tan)to translate the ephemeris into a satellite position and velocity vectorat a single point, for one satellite. Most of the computations alsorequire double precision, floating point processing.

Thus, the computational load for performing the traditional calculationis significant. The mobile device generally must therefore include ahigh-level processor capable of the necessary calculations, and suchprocessors are relatively expensive and consume large amounts of power.Portable devices for consumer use, e.g., a cellular phone or comparabledevice, are preferably inexpensive and operate at very low power. Thesedesign goals are inconsistent with the high computational load requiredfor GPS processing.

Further, the slow data rate from the GPS satellites is a limitation. GPSacquisition at a GPS receiver may take many seconds or several minutes,during which time the receiver circuit and processor of the mobiledevice must be continuously energized. Preferably, to maintain batterylife in portable receivers and transceivers such as mobile cellularhandsets, circuits are de-energized as much as possible. The long GPSacquisition time can rapidly deplete the battery of a mobile device. Inany situation and particularly in emergency situations, the long GPSacquisition time is inconvenient.

Assisted-GPS (“A-GPS”) has gained significant popularity recently inlight of stringent time to first fix (“TTFF”), i.e., first positiondetermination and sensitivity, requirements of the FCC E-911regulations. In A-GPS, a communications network and associatedinfrastructure may be utilized to assist the mobile GPS receiver, eitheras a standalone device or integrated with a mobile station or device.The general concept of A-GPS is to establish a GPS reference network(and/or a wide-area D-GPS network) including receivers with clear viewsof the sky that may operate continuously. This reference network mayalso be connected with the cellular infrastructure, may continuouslymonitor the real-time constellation status, and may provide data foreach satellite at a particular epoch time. For example, the referencenetwork may provide the ephemeris and the other broadcast information tothe cellular infrastructure. In the case of D-GPS, the reference networkmay provide corrections that can be applied to the pseudoranges within aparticular vicinity. As one skilled in the art would recognize, the GPSreference receiver and its server (or position determination entity) maybe located at any surveyed location with an open view of the sky.Typical A-GPS information may include data for determining a GPSreceiver's approximate position, time synchronization mark, satelliteephemerides, and satellite dopplers. Different A-GPS services may omitsome of these parameters; however, another component of the suppliedinformation is the identification of the satellites for which a deviceor GPS receiver should search.

However, the signal received from each of the satellites may notnecessarily result in an accurate position estimation of the handset ormobile device. The quality of a position estimate largely depends upontwo factors: satellite geometry, particularly, the number of satellitesin view and their spatial distribution relative to the user; and thequality of the measurements obtained from satellite signals. Forexample, the larger the number of satellites in view and the greater thedistances therebetween, the better the geometry of the satelliteconstellation. Further, the quality of measurements may be affected byerrors in the predicted ephemeris of the satellites, instabilities inthe satellite and receiver clocks, ionospheric and troposphericpropagation delays, multipath interference, receiver noise and RFinterference. A-GPS implementations generally rely upon providedassistance data to indicate which satellites are visible. Assistancedata may generally be provided to a mobile device as a function of anestimated or initial location of the mobile device. From such assistancedata, a mobile device will attempt to search for and acquire satellitesignals for the satellites included in the assistance data. If, however,satellites are included in the assistance data that are not measurableby the mobile device (e.g., the satellite is no longer visible, etc.),then the mobile device will waste time and considerable power attemptingto acquire measurements for the satellite.

In embodiments where an initial location of the handset or mobile deviceis determined as a function of the base station, cell, etc., situationsmay exist where this location is incorrectly known or is unknown (e.g.,when a Mobile Location Center (“MLC”) is employed as a service bureaufor multiple network operators). Thus, if the respective code phaseposition calculation does not know the initial location to within 100km, the position calculation for the mobile device may fail therebyhaving a significant impact on yield and accuracy for the MLC.

Accordingly, there is a need for a method and apparatus for geographiclocation determination of a device that would overcome the deficienciesof the prior art. Therefore, an embodiment of the present subject matterprovides a method for determining the location of a device. The methodcomprises the steps of receiving at a device plural signals from a firstplurality of satellites, determining an initial location of the deviceas a function of frequency information from the signals. A secondplurality of satellites may be determined as function of this initiallocation. Assistance data may then be transmitted to the device whichincludes information from the second plurality of satellites, and asecond estimated location of the device may be determined from theinformation from the second plurality of satellites. In anotherembodiment, another estimated location of the device may be determinedas a function of phase information from the signals. In anotherembodiment of the present subject matter, the location of the mobiledevice may be determined in an exemplary two-step process by utilizingfrequency shift (Doppler) information to calculate a coarse locationfrom such information, and then taking the coarse location as an inputand performing a more accurate location determination utilizing the codephase information.

In a further embodiment of the present subject matter, a system isprovided for determining the location of a device from signals receivedfrom a plurality of GNSS satellites. The system comprises a receiver forreceiving plural signals from a first plurality of satellites, circuitryfor determining an coarse or initial location of the device as afunction of frequency information from the signals, and circuitry fordetermining a second plurality of satellites as a function of the coarselocation. The system may also include a transmitter for transmittingassistance data to the device where the assistance data includesinformation from the second plurality of satellites and circuitry fordetermining another location of the device from the information from thesecond plurality of satellites. In another embodiment, the system mayinclude circuitry for determining a third estimated location of thedevice as a function of phase information from the signals. In yetanother embodiment of the present subject matter, the system maycomprise circuitry for determining the location of the mobile device inan exemplary two-step process by utilizing frequency shift (Doppler)information to calculate a coarse location from such information, andthen taking the coarse location as an input and performing a moreaccurate location determination utilizing the code phase information.

In an additional embodiment of the present subject, a method is providedfor determining a location of a device. The method may comprise thesteps of receiving at the device plural signals from a first pluralityof satellites and determining a first estimated location of the deviceas a function of frequency information from the signals. A secondestimated location of the device may be determined as a function of thefirst estimated location and as a function of phase information from thesignals. A second plurality of satellites may then be determined as afunction of the first or second estimated location. The method mayfurther include the steps of transmitting assistance data to the devicewhere the assistance data includes information from the second pluralityof satellites and determining a third estimated location of the devicefrom the information from the second plurality of satellites.

These embodiments and many other objects and advantages thereof will bereadily apparent to one skilled in the art to which the inventionpertains from a perusal of the claims, the appended drawings, and thefollowing detailed description of the embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a constellation of GPSsatellites.

FIG. 2 is a depiction of one method of selecting a second plurality ofsatellites according to an embodiment of the present subject matter.

FIG. 3 is a depiction of another method of selecting a second pluralityof satellites according to an embodiment of the present subject matter.

FIG. 4 is an algorithm according to one embodiment of the presentsubject matter.

FIG. 5 is another algorithm according to one embodiment of the presentsubject matter.

FIG. 6 is a schematic representation for implementing one embodiment ofthe present subject matter.

DETAILED DESCRIPTION

With reference to the figures where like elements have been given likenumerical designations to facilitate an understanding of the presentsubject matter, the various embodiments of a system and method forposition calculation of a mobile device are herein described.

The disclosure relates to methods and apparatuses for determininggeolocation using satellite signals and assistance data. The satellitesmay be considered as part of a Global Navigation Satellite System(“GNSS”), such as, but not limited to, the U.S. Global PositioningSystem (“GPS”). While the following description references the GPSsystem, this in no way should be interpreted as limiting the scope ofthe claims appended herewith. As is known to those of skill in the art,other GNSS systems operate, for the purposes of this disclosure,similarly to the GPS system, such as, but not limited to, the EuropeanSatellite project, Galileo; the Russian satellite navigation system,GLONASS; the Japanese Quasi-Zenith Satellite System (“QZSS”), and theChinese satellite navigation and positioning system called Beidou (orCompass). Therefore, references in the disclosure to GPS and/or GNSS,where applicable, as known to those of skill in the art, apply to theabove-listed GNSS systems as well as other GNSS systems not listedabove.

Generally wireless A-GPS devices or handsets have a low time to firstfix (“TTFF”) as the devices are supplied with assistance data from anexemplary communications network to assist in locking onto or acquiringsatellites quickly. Exemplary network elements that supply theassistance data may be a Mobile Location Center (“MLC”) or othercomparable network element. The MLC may generally be a node in awireless network that performs an A-GPS position calculation utilizingcode phases measured by a mobile device with a GPS receiver in thenetwork. In embodiments of the present subject matter, the respectiveposition calculation function (“PCF”) generally may be seeded with aninitial location of the mobile device within 100 km of the respectiveactual location to solve the GPS millisecond ambiguity problem. Themillisecond-ambiguity problem is generally a result of the knowledge ofthe code phase chips only within a predetermined time interval, e.g.,the present millisecond. If, however, a GPS receiver's location is knownwithin approximately 100 km, then the millisecond-ambiguity problem maybe resolved and an accurate location of a mobile device determined. TheMLC may generally determine A-GPS information utilizing an approximateor initial location of the device. Conventionally, this approximatelocation may be the location of the cell tower serving the device. TheMLC may then supply the device with the appropriate A-GPS assistancedata for the set of satellites in view from this conventional location.

This typical process performs well when the approximate locationpossesses a small uncertainty; however, in the absence of an approximatelocation or where the approximate location possesses a large uncertainty(e.g., an uncertainty measured in hundreds of kilometers) the possibleset of satellites may be quite large, and not all of the satellites inthis set may be measurable. As each satellite requires time andresources to provide assistance data therefor and signaling methodsoften limit the number of satellites for which signals may be provided,assistance data for only a subset of the set satellites may be providedto the mobile device.

Since A-GPS implementations generally rely upon the provided assistancedata to indicate which satellites are visible, the mobile deviceattempts to acquire only the satellite signals for the satellitesincluded in the assistance data. In the absence of a location estimate,a small number of the satellites included in the assistance data may bemeasurable for the mobile device resulting in no location fix or a poorquality location fix of the respective device.

Embodiments of the present subject matter may utilize a staged approachto determine a plurality or set of satellites to select and send to amobile device. In one embodiment of the present subject matter a widespread of satellites may be selected to ensure an even coverage over apredetermined location, such as, but not limited to, the entire planetor the entirety of the known area of the location estimate, e.g., cell,communications network, city, county, country, continent, etc.

After this selection of satellites, generally one of four outcomes mayoccur: (i) the device may be able to determine its respective locationwith adequate precision from available satellite measurements; (ii) thedevice may be able to provide a rough location estimate with apredetermined number of satellite measurements, but the locationestimate may not adequately precise or possesses a poor quality. Forexample, methods utilizing an earth-centered pseudo-measurement may beemployed with three satellite measurements, even with an inadequateprecision; standard A-GPS methods may then be employed to determineanother set of satellites for which signals may be provided to thedevice. The remaining outcomes may be that (iii) the device may be ableto provide one or two satellite measurements (in this instance, alocation estimate may not be determined, however, the satellitemeasurements may be utilized to select another plurality or set ofsatellites for which assistance data may be provided or that are morelikely to produce additional satellite measurements); and (iv) nosatellite measurements are obtained, whereby the aforementioned processmay be reattempted with a different set of satellites, or abandoned.

In the scenarios where a second plurality or set of satellites may bedetermined or selected, embodiments of the present subject matter mayprovide various methods for such a selection. For example, in oneembodiment of the present subject matter, a second plurality or set ofsatellites may be selected as a function of an intersection of thecoverage areas of the first plurality of satellites whereby thisintersection may be employed as the new reference location.

FIG. 2 is a depiction of one method of selecting a second plurality ofsatellites according to an embodiment of the present subject matter.With reference to FIG. 2, a first satellite 201 and a second satellite202 may be present in the first plurality or set of satellites. Ofcourse, any number of satellites may be present in the first pluralityor set of satellites and the depiction of two satellites in FIG. 2should not in any way limit the scope of the claims herewith as thisdepiction is provided for ease of description. The first satellite 201provides a first coverage area 211 projected upon the surface of theEarth 250. The second satellite 202 provides a second coverage area 212projected upon the surface of the Earth 250. An intersection area 220 ofthese two respective coverage areas 211, 212 may be employed as areference location or estimated location for which a second set orplurality of satellites is determined. In a further embodiment of thepresent subject matter, the coverage area may be extended or decreasedby a predetermined amount or area to thereby increase or reduce thenumber of satellites in the second plurality or set of satellites.

In another embodiment of the present subject matter, a second pluralityor set of satellites may be selected as a function of an occlusion maskdrawn from each measured satellite. FIG. 3 is a depiction of anothermethod of selecting a second plurality of satellites according to anembodiment of the present subject matter. With reference to FIG. 3,signals from a first satellite 301 and a second satellite 302 in a firstplurality of satellites may be measured by a device. The first pluralityof satellites may be any number or all of the satellites 301, 302, 303,304, 305 in a satellite constellation. In the scenario depicted by FIG.3, an occlusion mask 311, 312 may be drawn from any one or more measuredsatellites 301, 302 (it should be noted that in three-dimensions, theocclusion masks 311, 312 are conical). Satellites 304, 305 may then beremoved from a second plurality or set of satellites provided in futureassistance data if any one or more of the satellites are occluded by theEarth 350 from any one or more measured satellites 301, 302. Asillustrated, three satellites 305 are occluded by the Earth 350 fromboth measured satellites 301, 302, and four satellites 304 are occludedby the Earth 350 from one of the measured satellites 301 or 302. Thisillustration is exemplary only and should not in any way limit the scopeof the claims appended herewith. Any set or subset of the remainingsatellites 301, 302, 303 may then be selected for the second pluralityof satellites.

In a further embodiment of the present subject matter, the respectiveocclusion masks 311, 312 may be extended or decreased by a predeterminedamount or angle to thereby alter the conical mask to increase or reducethe number of satellites in the second plurality or set of satellites.For example, an exemplary occlusion mask may be extended if the mobiledevice is unable to measure satellites below a certain angle above thehorizontal. Additionally, an exemplary occlusion mask may be decreasedif the mobile device is able to measure satellites at a certain anglebelow the horizontal.

In yet another embodiment of the present subject matter, a secondplurality or set of satellites may be determined as a function ofDoppler measurements and/or the approximate or initial location of themobile device (e.g., within 1 to 2 km) calculated therefrom. In afurther embodiment, this location may also be utilized as an input tothe respective code phase position calculation to determine a moreaccurate location of the mobile device.

In these embodiments, the first and second plurality of satellites maybe mutually exclusive, that is, there may not be a satellite of thefirst plurality of satellites that is a member of the second pluralityof satellites; therefore, the associated assistance data would also bemutually exclusive. Of course, embodiments of the present subject mattermay include one or more common satellites in each of the first andsecond plurality or sets of satellites, especially in the instance wherethe mobile device was able to provide a measurement for the commonsatellite.

In one embodiment of the present subject matter, an initial location ofa mobile device may be calculated using Doppler measurements. In anotherembodiment, an exemplary method may also seed any one or all of theposition calculation functions with (0, 0, 0), that is, the center ofthe Earth in Earth-Centered Earth-Fixed (“ECEF”) coordinates. Anexemplary Doppler location calculation may then calculate the locationof a device within a predetermined distance (e.g., 5 to 10 km, less thanor equal to 100 km, etc.) to solve the millisecond ambiguity problemdescribed above. The Doppler location calculation may then be utilizedas the initial location for an exemplary code phase-based positioncalculation according to embodiments of the present subject matter. Onenon-limiting example of a Doppler position calculation is outlined inHill, J., “The Principle of a Snapshot Navigation Solution Based onDoppler Shift,” ION GPS 2001, 14^(th) International Technical Meeting ofthe Satellite Division of the Institute of Navigation, Sep. 11-14, 2001,the entirety of which is incorporated herein by reference.

Generally, Doppler shift occurs since a GPS signal travels at the speedof light. The rate of change of the range between a satellite and arespective receiver may expand or compress the wavelength effectivelymeasured by a receiver. For example, when a satellite approaches thereceiver, frequency may increase slightly and when the satelliterecedes, the frequency may decrease. Assuming a stationary receiver andutilizing Doppler measurements from one or more satellites, anapproximate or initial location of a mobile device may thus bedetermined. For example, the coarse acquisition (“C/A”) code is 1023bits long and repeats every millisecond. As an exemplary mobile devicemay measure the distance offset within the 1023 bits, the measurementsmay be ambiguous at the millisecond level. The number that the mobiledevice measures is the remainder part of the respective pseudorange orthe pseudorange modulo one millisecond.

The whole part of the pseudorange may then be determined in units of1023 bits and summed with the measured values. Generally, this is in therange of 70 as the travel time of the signal is in the order of 70milliseconds. The GPS chipping rate in seconds is generally 1.023·10⁶.The resolution of a 1023 bit C/A code in meters may be represented bythe following relationship:

$\begin{matrix}{{\frac{codelength}{GPSchippingrate} \cdot c} = 299792.458} & (1)\end{matrix}$

It follows that the resolution of one chip in meters may be representedby the following relationship:

$\begin{matrix}{\frac{{resolutionof}\; 1023\; {bitcode}}{1023} = 293.05226} & (2)\end{matrix}$

Therefore, the whole and part chips may be utilized to determine thepseudorange that represents the measured range modulo one millisecond.

To calculate the true pseudorange between the satellite and the receiverthe following steps may be utilized for each satellite: determine thelocation of the satellite using the time of receipt of the signals,determine the satellite clock correction using the time of receipt ofthe signals, and determine the distance between the estimated mobiledevice location and the location of the satellite and subtract thesatellite clock correction. If the mobile device clock correction isknown, then this value should also be subtracted as represented by thefollowing relationship:

range=√{square root over ((x _(s) −x _(r))²+(y _(s) −y _(r))²+(z _(s) −z_(r))²)}{square root over ((x _(s) −x _(r))²+(y _(s) −y _(r))²+(z _(s)−z _(r))²)}{square root over ((x _(s) −x _(r))²+(y _(s) −y _(r))²+(z_(s) −z _(r))²)}−satelliteclockcorr.−receiverclockcorr.   (3)

A fractional range from the whole and part GPS chips may be determinedutilizing the following relationship:

frange=(wholechips·resolutionofonechip)+(partchips·resolutionofonechip)  (4)

The number of whole units of 1023 bits in the range may now bedetermined utilizing the following relationship:

$\begin{matrix}{N = {{int}\left( {\frac{\left( {{range} - {frange}} \right)}{{resolutionof}\; 1023\; {bitcode}} + 0.5} \right)}} & (5)\end{matrix}$

The pseudorange may then be determined:

pseudorange=(N·resolutionof1023bitcode)+frange   (6)

In further embodiments of the present subject matter, to formulate aleast squares solution for a Doppler location estimate, the followingrelationship may be utilized:

Ax=b   (7)

The matrix A in Equation (7) may be represented by the followingrelationship:

$\begin{matrix}{A = {\begin{matrix}V_{0x} & V_{0y} & V_{0z} & {S_{0c}} \\V_{1x} & V_{1y} & V_{1z} & {S_{1c}} \\\vdots & \vdots & \vdots & \vdots \\V_{mx} & V_{my} & V_{0{mz}} & {S_{m\; c}}\end{matrix}}} & (8)\end{matrix}$

Each row in the above matrix A corresponds to each measured satellite.With reference to Equation (8), the first three terms for each rowrepresent the respective velocity of a satellite in the x, y and zdirections. The satellite velocity at time t may be determined using theephemeris. The final term in each row represents the matrix norm of asatellite location, e.g., the square root of the sum of the squares ofthe satellite location vector.

The x matrix in Equation (7) may be represented by the followingrelationship:

$\begin{matrix}{x_{n} = {\begin{matrix}U_{cxn} \\U_{cyn} \\U_{czn} \\{\Delta \; W_{n}}\end{matrix}}} & (9)\end{matrix}$

The vector represented by x_(n) is generally the unknown for which asolution should be found. With reference to Equation (9), the results ofa least squares process may generally provide a location in ECEFcoordinates, and ΔW_(n) represents any clock error in the respectivesolution. In one embodiment of the subject matter, x_(n) may be providedas the initial location estimate of a mobile device. However, in anotherembodiment, the ECEF coordinates (0, 0, 0) may also suffice.

With continued reference to Equation (7), the b matrix may berepresented by the following relationship:

B _(i)(x _(n))=W _(i) ∥S _(i) −U _(cn) ∥+S _(ic) ·V _(i) +ΔW_(n)(∥S_(ic) ∥−∥S _(ic) −U _(cn)∥)   (10)

Wi may be determined by the following relationship:

$\begin{matrix}{{Wi} = {\frac{\Delta \; F_{r}}{F_{t}}c}} & (11)\end{matrix}$

where ΔF_(r) represents the measured Doppler and F_(t) represents thetransmitted frequency of carrier signal L1 (e.g., 1.5754 GHz).

Upon construction of the A, x and b matrices, a least squares iterativeprocedure may be invoked by the following relationship:

x _(n+1)=(A ^(t) A)⁻¹ A ^(t) B(x _(n))   (12)

The resultant location determination may then be utilized as an input orseed for a second position calculation utilizing phase information suchas, but not limited to, C/A code phase information. The resultantlocation determination may also be utilized to determine a second set orplurality of satellites from which appropriate AGPS information may beprovided to an exemplary mobile device.

In one exemplary embodiment of the present subject matter, tests wereconducted over a full day of data from a stationary GPS receiver in opensky conditions. The following statistics show the horizontal error forthe respective GPS data:

TABLE 1 Horizontal Error Statistics for All Satellites Total Records155,043 Average 796.8 m Stdev 255.2 m Minimum 174.6 m Maximum 5691.4 m67% 827.9 m 95% 1136.6 m Yield For All Satellites 100.0

As shown above in the statistical data, the average error of the Dopplercalculated location was 796.8 m with a maximum error of 5691.4 m. Thus,the average error was considerably well within the 100 km required toprovide an input or seed a subsequent code phase position determination.

FIG. 4 is an algorithm 400 according to one embodiment of the presentsubject matter. With reference to FIG. 4, at step 410, plural signalsmay be received at a mobile device from a first plurality of satellites.In one embodiment of the present subject matter, the first plurality ofsatellites may be at least four. Of course, any appropriate number ofsatellites may comprise the first plurality, and such an example shouldnot limit the scope of the claims appended herewith. Exemplarysatellites may be a part of a Global Navigation Satellite System(“GNSS”) such as, but not limited to, Global Positioning System (“GPS”),Galileo, Global Navigation Satellite System (“GLONASS”), Quasi-ZenithSatellite System (“QZSS”), and combinations thereof. An exemplary devicemay be, but is not limited to, a cellular device, text messaging device,computer, portable computer, vehicle locating device, vehicle securitydevice, communication device, and wireless transceiver.

An estimated location of the device may be determined as a function offrequency information from the signals at step 420. In one embodiment ofthe present subject matter, the estimated location may be determined asa function of coordinates for origin in an ECEF coordinate system. Inyet another embodiment, the frequency information may include Dopplershift information. At step 430, a second plurality of satellites may bedetermined as a function of the estimated location. Assistance data maythen be transmitted to the device where the assistance data includesinformation from the second plurality of satellites at step 440. At step450, another location of the device may then be determined from thisinformation. In one exemplary embodiment, the first and second pluralityof satellites may be mutually exclusive. Of course, any number ofsatellites in the first and second plurality of satellites may be commontherebetween. In another embodiment, a third location of the device maybe determined as a function of phase information from the signals.Exemplary phase information may include C/A code phase information.

FIG. 5 is another algorithm 500 according to one embodiment of thepresent subject matter. With reference to FIG. 5, at step 510, pluralsignals may be received at a mobile device from a first plurality ofsatellites. An estimated location of the device may be determined as afunction of frequency information from the signals at step 520, and atstep 530 a second estimated location determined as a function of thefirst estimated location and phase information from the signals. At step540, a second plurality of satellites may be determined as a function ofany one of the first or second estimated locations. Assistance data maythen be transmitted to the device where the assistance data includesinformation from the second plurality of satellites at step 550. At step560, another location of the device may then be determined from thisinformation. In one exemplary embodiment, the first and second pluralityof satellites may be mutually exclusive. Of course, any number ofsatellites in the first and second plurality of satellites may be commontherebetween.

FIG. 6 is a schematic representation for implementing one embodiment ofthe present subject matter. With reference to FIG. 6, a satellite system610 may communicate with a ground system 620. An exemplary satellitesystem 610 may be a GNSS such as, but not limited to, GPS, Galileo,GLONASS, QZSS, and combinations thereof. The ground system 620 mayinclude a cellular network having a location center 621. The locationcenter 621 may be a Mobile Location Center (MLC) or another networkcomponent such as a central office configured to communicate with atelecommunication network 622 and at least one base station 623. In oneembodiment of the present subject matter, a device 624 may communicatewith the base station 623 to acquire GPS assistance data. For example,the location center 621 may or may not receive a preliminary estimate ofthe device's location or boundary thereof on the basis of the device'sserving or neighboring cell site, sector, network boundary, or otherarea. Further the preliminary estimate may be a function of frequencyinformation as discussed above. The location center 621 may alsodetermine a plurality of satellites as a function of this boundary orregion and determine whether any one or more of these plural satellites,while operational, are not visible by the device 624 for some reason.The location center 621 may also receive satellite information from GPSsatellites. The satellite information may include the satellite'sbroadcast ephemeris information of the broadcasting satellite, that ofall satellites, or that of selected satellites. Further, the locationcenter 621 may manipulate the assistance data to prevent the device 624from searching and attempting to acquire signals from one or moresatellites. This information may then be transmitted or relayed to thedevice 624 and utilized for location determination. The location center621 may relay the information back to the device 624 or use theinformation, either singularly or along with some preliminary estimationof the device's location, to assist the device 624 in a geographiclocation determination. In another embodiment, any one or plural stepsillustrated in FIGS. 4 and 5 may be implemented at the location center621 and communicated to the device 624. Of course, the estimatedlocation of the device 624 may be determined as a function of additionalsignals provided by the network 622. Exemplary devices may be, but arenot limited to, a cellular device, text messaging device, computer,portable computer, vehicle locating device, vehicle security device,communication device, and wireless transceiver.

In another embodiment, the device 624 may acquire GPS informationdirectly from plural satellites in the satellite system 610. Forexample, the device 624 may include a receiver for receiving pluralsignals from a first plurality of satellites and respective circuitryfor determining an estimated location thereof as a function of frequencyinformation from the signals. Exemplary frequency information may be,but is not limited to, Doppler shift information. The device 624 mayalso include circuitry for determining a location thereof as a functionof the estimated location and as a function of phase information fromthe signals. Exemplary phase information may include C/A code phaseinformation. The determined location may also be a function ofcoordinates for origin in an ECEF coordinate system. Of course, thedevice 624 may receive assistance data from the location center 621 thatmay include information from a second plurality of satellites. Thedevice 624 may also comprise circuitry for determining another locationthereof from this information. Of course, the first and second pluralityof satellites may be mutually exclusive, or any number of satellites inthe first and second plurality of satellites may be common therebetween.

As shown by the various configurations and embodiments illustrated inFIGS. 1-6, a method and system for position calculation of a mobiledevice have been described.

While preferred embodiments of the present subject matter have beendescribed, it is to be understood that the embodiments described areillustrative only and that the scope of the invention is to be definedsolely by the appended claims when accorded a full range of equivalence,many variations and modifications naturally occurring to those of skillin the art from a perusal hereof.

1. A method for determining a location of a device, the methodcomprising: (a) receiving at said device plural signals from a firstplurality of satellites; (b) determining a first estimated location ofsaid device as a function of frequency information from said signals;(c) determining a second plurality of satellites as a function of saidfirst estimated location; (d) transmitting assistance data to saiddevice, said assistance data including information from said secondplurality of satellites; and (e) determining a second estimated locationof said device from said information from said second plurality ofsatellites.
 2. The method of claim 1 further comprising the step ofdetermining a third estimated location of said device as a function ofphase information from said signals.
 3. The method of claim 1 whereinsaid first plurality of satellites is at least four.
 4. The method ofclaim 1 wherein the satellites are part of a Global Navigation SatelliteSystem (“GNSS”).
 5. The method of claim 4 wherein the GNSS is selectedfrom the group consisting of: Global Positioning System (“GPS”),Galileo, Global Navigation Satellite System (“GLONASS”), andQuasi-Zenith Satellite System (“QZSS”).
 6. The method of claim 1 whereinthe device is selected from the group consisting of: cellular device,text messaging device, computer, portable computer, vehicle locatingdevice, vehicle security device, communication device, and wirelesstransceiver.
 7. The method of claim 1 wherein said first estimatedlocation is determined as a function of coordinates for origin in anEarth-Centered Earth-fixed (“ECEF”) coordinate system.
 8. The method ofclaim 1 wherein said first and second plurality of satellites aremutually exclusive.
 9. The method of claim 1 wherein said frequencyinformation includes Doppler shift information.
 10. The method of claim2 wherein said phase information includes coarse acquisition (“C/A”)code phase information.
 11. A system for determining the location of adevice from signals received from a plurality of Global NavigationSatellite System (“GNSS”) satellites comprising: (a) a receiver forreceiving plural signals from a first plurality of satellites; (b)circuitry for determining a first estimated location of said device as afunction of frequency information from said signals; (c) circuitry fordetermining a second plurality of satellites as a function of said firstestimated location; (d) a transmitter for transmitting assistance datato said device, said assistance data including information from saidsecond plurality of satellites; and (e) circuitry for determining asecond estimated location of said device from said information from saidsecond plurality of satellites.
 12. The system of claim 11 furthercomprising circuitry for determining a third estimated location of saiddevice as a function of phase information from said signals.
 13. Thesystem of claim 11 wherein said first plurality of satellites is atleast four.
 14. The system of claim 11 wherein the GNSS is selected fromthe group consisting of: Global Positioning System (“GPS”), Galileo,Global Navigation Satellite System (“GLONASS”), and Quasi-ZenithSatellite System (“QZSS”).
 15. The system of claim 11 wherein the deviceis selected from the group consisting of: cellular device, textmessaging device, computer, portable computer, vehicle locating device,vehicle security device, communication device, and wireless transceiver.16. The system of claim 11 wherein said first estimated location is afunction of coordinates for origin in an Earth-Centered Earth-fixed(“ECEF”) coordinate system.
 17. The system of claim 11 wherein saidfirst and second plurality of satellites are mutually exclusive.
 18. Thesystem of claim 11 wherein said frequency information includes Dopplershift information.
 19. The system of claim 12 wherein said phaseinformation includes coarse acquisition (“C/A”) code phase information.20. A method for determining a location of a device, the methodcomprising: (a) receiving at said device plural signals from a firstplurality of satellites; (b) determining a first estimated location ofsaid device as a function of frequency information from said signals;(c) determining a second estimated location of said device as a functionof said first estimated location and as a function of phase informationfrom said signals; (d) determining a second plurality of satellites as afunction of said first or second estimated location; (e) transmittingassistance data to said device, said assistance data includinginformation from said second plurality of satellites; and (f)determining a third estimated location of said device from saidinformation from said second plurality of satellites.
 21. The method ofclaim 20 wherein said first plurality of satellites is at least four.22. The method of claim 20 wherein the satellites are part of a GlobalNavigation Satellite System (“GNSS”).
 23. The method of claim 22 whereinthe GNSS is selected from the group consisting of: Global PositioningSystem (“GPS”), Galileo, Global Navigation Satellite System (“GLONASS”),and Quasi-Zenith Satellite System (“QZSS”).
 24. The method of claim 20wherein the device is selected from the group consisting of: cellulardevice, text messaging device, computer, portable computer, vehiclelocating device, vehicle security device, communication device, andwireless transceiver.
 25. The method of claim 20 wherein said firstestimated location is determined as a function of coordinates for originin an Earth-Centered Earth-fixed (“ECEF”) coordinate system.
 26. Themethod of claim 20 wherein said first and second plurality of satellitesare mutually exclusive.
 27. The method of claim 20 wherein saidfrequency information includes Doppler shift information.
 28. The methodof claim 20 wherein said phase information includes coarse acquisition(“C/A”) code phase information.